The invention relates to a tire with radial carcass reinforcement, and more particularly to a tire intended to be fitted on vehicles bearing heavy loads and traveling at sustained speed, such as, for example, lorries, tractors, trailers or highway buses.
The carcass reinforcement is anchored on either side to at least one bead wire, and is surmounted radially by a crown reinforcement comprising at least two so-called working plies which are superposed and formed of cords or cables which are parallel within each ply and are crossed from one ply to the next, forming angles of at most 45xc2x0 in absolute value with the circumferential direction of the tire. It also generally comprises a ply of metal cords or cables of low extensibility which form an angle of between 45xc2x0 and 90xc2x0 with the circumferential direction, this ply, called a triangulation ply, being radially located between the carcass reinforcement and the first, so-called working, crown ply. The triangulation ply forms with the two working piles a triangulated reinforcement which has little deformation under the different stresses to which it is subjected, the essential role of the triangulation ply being to take up the transverse compressive forces to which all the reinforcements in the zone of the crown of the tire are subjected.
Certain current tires, referred to as xe2x80x9chighwayxe2x80x9d tires, are intended to travel at high speed and on increasingly long journeys, owning to the improvement in road networks and the growth in the motorway networks throughout the world. All the conditions under which such a tire is required to travel without doubt permits an increase in the number of kilometers traveled, the wear of the tire being less; on the other hand, the endurance of the latter, and in particular of the crown reinforcement, is greatly impaired.
The stresses existing at the level of the crown reinforcement, and more particularly the shearing stresses between the two working crown piles, combined with a non-negligible increase in the operating temperature at the ends of the shortest working ply, result in the appearance and propagation of cracking of the rubber at said ends, despite the presence of a thickened layer of rubber at the junction of the edges of the working crown plies. The same problem exists in the case of edges of two plies having reinforcement elements, crossed from one ply to the other ply, said other ply not necessarily being radially adjacent to the first.
In order to overcome the above disadvantages and to improve the life of the crown reinforcement of the type of tire in question, a certain number of prior patents claim solutions relating to the structure and quality of the layers and/or profiled elements of rubber mixes which are arranged between and/or around the ends of working plies, and more particularly the ends of the shortest working ply.
French Patent 1 389 428, to improve the resistance to degradation of the rubber mixes located in the vicinity of the edges of the crown reinforcement, advocates the use, in combination with a tread of low hysteresis, of a rubber profiled element covering at least the sides and the marginal edges of the crown reinforcement and formed of a rubber mix of low hysteresis.
French Patent 2 222 232, in order to avoid separations between crown reinforcement plies, teaches coating the ends of the reinforcement with a pad of rubber, the Shore A hardness of which differs from that of the tread surmounting said reinforcement, and is greater than the Shore A hardness of the profiled element of rubber mix arranged between the edges of crown reinforcement plies and the carcass reinforcement.
U.S. Pat. No. 5,154,217 uses a different unit of measurement, and claims arranging between the ends of two plies, in the extension of the ply between said two plies, a pad of rubber mix, the elasticity modulus of which at 100% elongation is greater than the modulus of the same name of the tread.
To effect joining of the respective two edges of two crown reinforcement plies, French Patent 2 298 448 discloses the use of so-called shearing rubbers between said edges, of a high Shore A hardness and elasticity modulus at 100%, this use being combined with the use of anisotropic rubber strips arranged between the edges of the crown reinforcement and the carcass reinforcement.
The same applies to the case of joining of the crown reinforcement plies described in French Patent 2 499 912, the lateral part of the rubber layer arranged between the two main plies of the crown reinforcement being formed of a rubber mix of high Shore hardness.
Other solutions have been considered for improving the resistance to separation of the crown reinforcement plies, which solutions consist in coating at least one end of the axially least wide ply in at least one layer of rubber mix, whether or not reinforced by reinforcement elements. In the case of a single layer, the latter is advantageously turned up over the end of the ply, as described and illustrated, for example, in document FR 1 226 595, where the layer is reinforced by very fine metal wires, or in document JP 266 703, the protective layer being constituted solely of rubber of a high elasticity modulus, or alternatively in French Patent 2 671 516. European application EP 0 785 095 A1 relates to a tire for heavy vehicles, and more particularly to a crown reinforcement for such a tire, formed of at least three plies, two of said plies being composed of crossed elements. Said application teaches providing the edges of the shortest crown ply with an edging rubber, the tensile modulus of which at 100% relative elongation is greater than that of the coating mix for the reinforcement elements, whereas the layer of rubber mix arranged between the edges of plies has a lower modulus than the modulus of said coating mix.
The various structures listed above did not provide a completely satisfactory solution under conditions of travel which are highly disadvantageous to the tire.
According to the invention, the tire having a radial carcass reinforcement, which is anchored within each bead to at least one bead wire, and is surmounted by a crown reinforcement comprising at least two plies of reinforcement elements, which are parallel to each other within each ply and are crossed from one ply to the next, said two plies being of unequal axial widths, each edge of the ply which is axially less wide of at least a pair of crossed plies being separated from the axially widest ply of the same pair by a profiled element P of rubber mix, the axially outer end of which is located at a distance from the equatorial plane of the tire which is at least equal to the distance between said plane and the end of the widest ply, said profiled element P itself being separated from the liner C contacting the reinforcement elements of the least wide ply by an edging rubber G, the axially outer end of which is located at a distance from said plane which is at least equal to half the width of the least wide ply, is characterized in that said profiled element P, said edging rubber G and said liner C have respectively secant moduli of elasticity under tension at 10% relative elongation MP, MG, MC such that MCxe2x89xa7MG greater than MP, the thickness of the edging rubber G being at least equal to 15% of the total thickness of rubber mix between generatrices of cables respectively of the two plies.
The edging rubber G will have its axially inner end preferably located at a distance from the equatorial plane which is at most equal to the distance between said plane and the axially inner end of said profiled element P.
Said edging rubber G may be turned up around the edge of the least wide crown ply of the two crossed plies in question, so as to coat completely the ends of the reinforcement elements forming said least wide crown reinforcement ply.
The thickness of the profiled element P, measured at the end of the least wide ply of the two plies in question, will preferably be between 30% and 80% of the total thickness of rubber mix between generatrices of cables respectively of the two plies: a thickness of less than 30% does not make it possible to obtain convincing results, and a thickness of more than 80% is useless with regard to the improvement in the resistance to separation between plies and is disadvantageous from the point of view of cost.
The characteristics of the invention will be better understood with reference to the following description, which refers to the drawings, which illustrate examples of embodiment in non-limitative manner.